1. Field of the Invention
The invention relates to a digital-to-analog current converting circuit, and more particularly, to a current-storing/reproducing digital-to-analog current converting circuit including a current storing/reproducing module.
2. Description of the Prior Art
An OLED (Organic Light Emitting Device, OLED) display can be designed as a thin, flat panel display device. The OLED display can be found in a plethora of electronic goods, ranging from notebook computers and digital cameras, to flight avionics and medical diagnostic tools. OLEDs offer crisp, high-resolution images, and have the primary advantage of offering relatively low power-consumption while still maintaining good color contrast and screen refresh rates. In recent years, many OLED manufacturing procedures include a LTPS TFT (Low Temperature Poly Silicon Thin Film Transistor) processing technique.
For achieving advantages of power saving, integrity, and cost effectiveness, more OLED systems adopt the digital type as an input data type so that the digital-to-analog converter should be involved in the data driver. In addition, the brightness of the OLED display is controlled by current. Therefore, the digital-to-analog process should be achieved by a digital-to-analog current converting to convert digital data into an analog current signal. The corresponding pixel is also a current-driving pixel. Please refer to FIG. 1, which is a functional block diagram of a prior-art data driver 10. The data driver 10 corresponds to a pixel 20 of a display device. The data driver 10 includes a level shifter 12, a latch 14, a shift register 16, and a digital-to-analog current converter 18. The level shifter 12 is used to adjust the potential levels of a received digital signal (a6-bit digital signal), and the latch 14 is electrically connected to the level shifter 12 for buffering the digital signal. The latch 14 can temporarily store the 6-bit the digital signal so that the latch 14 is a 6-bit latch. The shift register 16 can be used to generate a shift-register signal to transmit the digital signal to the level shifter 12 at one time. Afterwards, the level shifter 12 will execute the potential-level adjusting and buffering functions and transmit the digital signal to the latch 14. The digital-to-analog current converter 18 is connected to the latch 14 for receiving the digital signal outputted from the latch 14. The digital-to-analog current converter 18 can be used to transform the digital data into an analog current signal and to output the analog current signal to a data line 19. According to the amplitude of the analog current signal, the gray colors of the display panel can be determined.
Taking a display panel with 4-bit input digital data as an example, J. Kanicki et.al. (U. of Michigan, USA) has disclosed a simple digital-to-analog current converter installed with a set of TFTs (Thin Film Transistors) with width-to-length ratio assigned as 1:2:4:8 and a current source to generate 16 current gray scales, I. Please refer to FIG. 2, which is a schematic diagram of an embodiment of a prior-art digital-to-analog current converter 18 as shown in FIG. 1. The digital-to-analog current converter 18 is composed of a plurality of transistors T0-T4. Due to that the 16 current gray scales rely on 4 (1:2:4:8) TFTs T1-T4, any fluctuation of threshold potential level and mobility in each TFT will generate significant variation to affect the current gray scales. Furthermore, the quality of the corresponding panel will be influenced. In addition, because the output impedance of the digital-to-analog current converter 18 is not high enough, the output potential level will be affected by a current flow passing the digital-to-analog current converter 18. Therefore, when the digital-to-analog current converter 18 is connected to the corresponding pixel, the output current may not be a stable 16 gray-scale current.
It is therefore a primary objective of the claimed invention to provide a current storing/reproducing digital-to-analog current converting circuit to conduct a duplicate current signal in a reproducing/sustaining status to solve the above-mentioned problems.
According to the claimed invention, a digital-to-analog current converting circuit comprises a digital-to-analog current converter for transforming a digital signal into an analog current signal; a current storing/reproducing module for storing a predetermined voltage required for conducting the analog current signal in a transforming/storing status and for conducting a duplicate current signal to a data line in a reproducing/sustaining status; and a control circuit electrically connected between the digital-to-analog current converter and the current storing/reproducing module for switching the digital-to-analog current converting circuit between the transforming/storing status and the reproducing/sustaining status.
According to the claimed invention, a digital-to-analog current converting circuit used in a display device is disclosed. The digital-to-analog current converting circuit comprises a current-steering digital-to-analog current converter for transforming a received digital signal into an analog current signal; a plurality of current storing/reproducing modules, each current storing/reproducing module used for storing a predetermined voltage required for conducting the analog current signal in a corresponding transforming/storing status and for conducting a duplicate current signal to the corresponding data line in a corresponding reproducing/sustaining status; and a plurality of control circuits respectively electrically connected between the digital-to-analog current converter and the plurality of current storing/reproducing modules for switching the plurality of current storing/reproducing modules between the transforming/storing status and the reproducing/sustaining status; wherein each duplicate current signal generated by each current storing/reproducing module is almost equal to the analog current signal.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.